One example of a conventional flat case is a case for video tape cassettes. Known examples of cases for housing shells in which magneto-optical discs are held include hinged hard cases. Such hinged hard cases may be opened and closed to allow access to the disc housed therein.
Such known cases of the prior art for housing disc-shaped objects and the like are typically comprised of two or three components. In this regard, since hinges are also typically required, conventional cases for flat objects tend to be complex and expensive, thus raising the selling price of the magneto-optical discs housed therein and making them somewhat "over packaged".
Therefore, in order to eliminate these problems, a previously proposed inexpensive flat case is described in Japanese Utility Model Laid-Open No. 1-154190 and includes overlapped tabs which are inwardly folded back at the ends of a thin thermoplastic sheet which exhibits some rigidity. More specifically, the overlapping tabs are formed on both sides of fold lines provided nearly in the center of a rigid thermoplastic sheet having a thickness of roughly 0.1 mm or more by inwardly folding back each of the opposing sides of the sheet a plurality of times. Opposed folded-back edges are thereby formed by having each of the overlapping tabs oppose each other. The overlapped tabs are then collectively adhered to one another by ultrasonic welding. A shell holding a disc-shaped recording medium such as a magneto optical disc (more simply referred to as a "shell") may then be inserted into and removed from an opening formed in the opposed sides of the folding lines.
However, although insertion and removal of the shell is relatively easy when using the conventional flat case as described above (i.e., since the shell is free inside the flat case), the shell can easily fly out of the flat case during transport, thus making it dangerous.